1. Field of the Invention
The present invention relates to a cryopump.
2. Description of the Related Art
A cryopump is a vacuum pump that captures and pumps gas molecules by condensing or adsorbing molecules on a cryopanel cooled to an extremely low temperature. A cryopanel is generally used to achieve a clean vacuum environment required in a semiconductor circuit manufacturing process.
A cryopump having a heat shield plate provided with a cutout for enabling an inflow of gas molecules and an additional shield for preventing radiation heat from entering through the cutout is disclosed for example in Patent Documents 1 and 2. In this cryopump, a process gas entering into a space between the heat shield plate and the cryopump container is received inside the heat shield plate and is condensed and pumped on the second stage panels. Therefore, the heat transfer from the cryopump container to the heat shield plate via the process gas is reduced, and thereby the temperature rise of the heat shield plate and deterioration of cryopumping performance are mitigated or prevented.    [Patent Document 1] WO2005/050018    [Patent Document 2] JP2007-132273
The gas molecules condensed on a cryopanel accumulate as frost and/or ice. When it grows up and contacts to the heat shield plate having a higher temperature, re-vaporization starts and the cryopump does not perform further evacuation. An amount of condensed gas on the cryopanels before the contact significantly influences the maximum amount of condensed gas in the cryopump. In case that the heat shield plate has a cutout and an additional shield, the additional shield may restrict the maximum amount of condensed gas due to the contact between the additional shield and the accumulated frost on the second stage panels.
Also, the ice may concentrate on the top panel in the second stage panels as a gas enters mainly from an opening at the top end of the heat shield plate. The thick ice layer deposited on the top panel may have a higher temperature at its surface than the top panel due to a temperature gradient in the ice layer. The surface temperature of the ice layer also influences the maximum amount of condensed gas in the cryopump. When the vapor pressure on the surface of the ice layer exceeds the degree of vacuum to be attained, vaporization from the ice layer is predominant over gas condensation from ambient atmosphere to the ice layer, and hence further evacuation cannot be performed.